In order to obtain high-strength components of a grade of 1180 MPa or higher used for automobile components or the like with excellent dimensional precision, in recent years, a technology (hereinafter, referred to as hot stamping forming) for realizing high strength of a formed product by heating a steel sheet to an austenite range, performing pressing in a softened and high-ductile state, and then rapidly cooling (quenching) in a press die to perform martensitic transformation has been developed.
In general, a steel sheet used for hot stamping contains a lot of C component for securing product strength after hot stamping and contains austenite stabilization elements such as Mn and B for securing hardenability when cooling a die. However, although the strength and the hardenability are properties necessary for a hot stamped product, when manufacturing a steel sheet which is a material thereof, these properties are disadvantageous, in many cases. As a representative disadvantage, with a material having such a high hardenability, a hot-rolled sheet after a hot-rolling step tends to have an uneven microstructure in locations in hot-rolled coil. Accordingly, as means for solving unevenness of the microstructure generated in a hot-rolling step, performing tempering by a batch annealing step after a hot-rolling step or a cold-rolling step may be considered, however, the batch annealing step usually takes 3 or 4 days and thus, is not preferable from a viewpoint of productivity. In recent years, in normal steel other than a material for quenching used for special purposes, from a viewpoint of productivity, it has become general to perform a thermal treatment by a continuous annealing step, other than the batch annealing step.
However, in a case of the continuous annealing step, since the annealing time is short, it is difficult to perform spheroidizing of carbide to realize softness and evenness of a steel sheet by long-time thermal treatment such as a batch treatment. The spheroidizing of the carbide is a treatment for realizing softness and evenness of the steel sheet by holding in the vicinity of an Ac1 transformation point for about several tens of hours. On the other hand, in a case of a short-time thermal treatment such as the continuous annealing step, it is difficult to secure the annealing time necessary for the spheroidizing. That is, in a continuous annealing installation, about 10 minutes is the upper limit as the time for holding at a temperature in the vicinity of the Ac1, due to a restriction of a length of installation. In such a short time, since the carbide is cooled before being subjected to the spheroidizing, the steel sheet has an uneven microstructure in a hardened state. Such partial variation of the microstructure becomes a reason for variation in hardness of a hot stamping material.
Currently, in a widely-used hot stamping formation, it is general to perform quenching at the same time as press working after heating a steel sheet which is a material by furnace heating, and by heating in a heating furnace evenly to an austenitic single phase temperature, it is possible to solve the variation in strength of the material described above. However, a heating method of a hot stamping material by the furnace heating has poor productivity since the heating takes a long time. Accordingly, a technology of improving productivity of the hot stamping material by a short-time heating method by an electrical-heating method is disclosed. By using the electrical-heating method, it is possible to control temperature distribution of a sheet material in a conductive state, by modifying current density flowing to the same sheet material (for example, Patent Document 1).
If the temperature variation exists in the steel sheet for hot stamping by partially heating the steel sheet, the microstructure of the steel sheet does not significantly change from the microstructure of the base material at a non-heated portion. Accordingly, the hardness of the base material before heating becomes directly the hardness of the component. However, as mentioned above, the material which is subject to the cold-rolling after hot-rolling and the continuous annealing has a variation in the strength as shown in FIG. 1, and thus, the non-heated portion has a large variation in the hardness. Accordingly, there is a problem in that a formed component has a variation in the collision performance and the like and thus it is difficult to manage the precision of the quality of the component.
In addition, in order to solve the variation in the hardness, when heating at a temperature equal to or higher than Ac3 so as to be an austenite single phase in an annealing step, a hardened phase such as martensite or bainite is generated in an end stage of the annealing step due to high hardenability by the effect of Mn or B described above, and the hardness of a material significantly increases. As the hot stamping material, this not only becomes a reason for die abrasion in a blank before stamping, but also significantly decreases formability or shape fixability of the non-heated portion. Accordingly, if considering not only a desired hardness after hot stamping quenching, formability or shape fixability of the non-heated portion, a preferable material before hot stamping is a material which is soft and has small variation in hardness, and a material having an amount of C and hardenability to obtain desired hardness after hot stamping quenching. However, if considering manufacturing cost as a priority and assuming the manufacture of the steel sheet in a continuous annealing installation, it is difficult to perform the control described above by an annealing technology of the related art.
Accordingly, if a formed body is obtained by hot stamping a steel sheet which is heated so as to make a heated portion and a non-heated portion exist in the steel sheet, there is a problem in that the formed body one-by-one includes a variation in hardness at the non-heated portion.